GTPases govern critical cellular processes such as signal transduction, transcription, mitosis, cell shape and cell movement by acting as molecular switches that regulate activity of protein partners. The functions of small GTPases such as Ras and RhoA are relatively well understood, but there exists a variety of large molecular weight GTPases whose functions are poorly characterized. GTPBP1 and GTPBP2 are unique, large GTPases, distantly related to translation elongation factor EF1-alpha. Their biochemical and biological functions, however, are completely unknown. We have made a breakthrough by discovering that GTPBP1 and GTPBP2 interact with Smad proteins, the principal signal transducers for the Transforming Growth Factor-[unreadable], (TGF[unreadable]) superfamily. GTPBP2 in particular enhances signaling by the BMP and nodal/activin branches of TGF[unreadable] signaling in embryo and cultured cell assays. The GTPBP genes are expressed during early Xenopus embryogenesis and during subsequent organogenesis of somites, blood, heart and nervous system. Blocking endogenous GTPBP 1 or 2 in developing embryos with antisense oligonucleotides disrupts mesoderm differentiation, body patterning and organogenesis. In this grant, we propose to investigate how GTPBP 1 and 2 regulate TGF[unreadable] signaling and early vertebrate development. Aim 1 will define the specificity and molecular basis of interaction between GTPBPs and Smads. Aim 2 will investigate how GTPBPs influence BMP and nodal/activin signaling, using differentiation and reporter gene assays. Aim 3 will investigate the developmental roles of the GTPBPs in Xenopus embryos by gain and loss of function, and we will begin a search for links between GTPBPs and upstream pathways. The TGF[unreadable] superfamily controls cell growth, cell death, differentiation and development, and abnormal TGF[unreadable] signaling causes cancer, hypertension, fibroses and other diseases and birth defects. Likewise, abnormalities in GTPases cause cancer, a wide range of diseases, and birth defects. Thus, TGF[unreadable] signaling proteins and GTPases are popular drug targets. Whether dysfunctional GTPBP1/2 affect disease is not known, but a chromosomal translocation of GTPBP2 has recently been identified in glioblastoma, a nearly incurable brain cancer. Our studies will provide crucial information about GTPBPs which may eventually translate into clinical treatments for diseases caused by abnormal TGF[unreadable] or GTPBP signaling. [unreadable] [unreadable] [unreadable]